1. Field of the Invention
The present invention relates to a technique of detecting a symbol timing and a preamble in a burst communication system.
2. Description of Related Art
In the field of wireless communication, a burst communication system which treats signal data that is composed of a packet or the like under a particular criterion as one unit (burst or packet) is used. Such a packet or burst includes a preamble signal (which is also referred to hereinafter simply as a preamble) at its head. The preamble is composed of a synchronization portion for a frequency in use and a symbol timing synchronization portion for detecting the head of a transmitted symbol. When receiving such a packet, it is necessary to perform synchronization processing such as detection of a symbol timing or the like using a preamble. Because a preamble signal does not contain data to be transmitted (which is also referred to hereinafter as a payload), it is required to minimize the preamble for the synchronization processing in order to reduce the circuit size and increase the speed of the synchronization processing. For example, a preamble of an OFDM (Orthogonal Frequency Division Multiplex) packet is composed of a plurality of short symbols which are attached before a payload.
For the detection of a preamble in a receiver in OFDM, a technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-039597 is known, for example. The technique seeks the correlation between a received OFDM packet and a prestored fixed preamble pattern and detects a peak value in the correlation result. Then, the technique measures the number of times when the peak value exceeds a boundary detection threshold, which is described later, and the cyclic period of the peak value to thereby determine the position of a short symbol. Using the determined position of the short symbol, when the correlation result of the position at which the next peak should appear does not exceed the boundary detection threshold, the technique detects the peak position which is immediately preceding the above position and which exceeds the boundary detection threshold as a boundary between a short symbol and a data area.
Although the boundary detection threshold is a fixed value in other techniques, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-039597 uses a value that is obtained by multiplying a peak value in a correlation result by a coefficient smaller than 1 as the boundary detection threshold. It is thereby possible to detect a preamble without fail even when the level of a fixed received signal changes, thereby enabling the detection which does not depend on the receiving environment.
Because such a preamble detection method detects a preamble by confirming the periodicity of a symbol using a symbol timing, it is important to detect a symbol timing accurately.
As a UWB (Ultra Wide Band) communication method, there is MB-OFDM (Multiband-Orthogonal Frequency Division Multiplex). MB-OFDM is described in detail in Standard ECMA-368 High Rule Ultra Wideband PHY and MAC Standard (Http://www.ecma-international.org/publications/files/EC MA-ST/ECMA-368.pdf). The MB-OFDM system performs communication using frequency hopping among a plurality of frequency bands in order to achieve broadband communication with a low-transmission power, which are the characteristics of the UWB communication. The frequency hopping is one form of spread spectrum and it is a method of transmitting signals by switching a carrier wave frequency within a certain communication band at a given time interval, using a hopping sequence known to both transmitter and receiver.
In MB-OFDM, a preamble signal is also transmitted via frequency hopping. FIG. 17 shows an example of a preamble signal in a MB-OFDM packet. The preamble signal is composed of 24 symbols (S0 to S23) and transmitted via hopping among three frequency bands (frequency band 1, frequency band 2 and frequency band 3). A receiver synchronizes a received frequency with frequency hopping according to the hopping sequence, receives the symbol over three frequency bands and demodulates it. In order to synchronize with frequency hopping, it is necessary to make sure to detect a preamble which is transmitted via frequency hopping. For this reason, a receiver fixes a received frequency to a standby frequency band (e.g. the frequency band 1 shown in FIG. 17) at the start of receiving to establish symbol timing synchronization and then detects a preamble.
After detecting a preamble, the receiver starts frequency hopping and performs initial acquisition such as AGC (Automatic Gain Control), AFC (Automatic Frequency Control), and frame synchronization in the illustrated 24 symbols, using the preamble signal which is received after that. In the example of FIG. 17, if the number of symbols which is required for AGC, AFC and frame synchronization is nineteen (S5 to S23), the symbols which are available for symbol timing synchronization are only two: S0 and S3.
Thus, in the MB-OFDM system which performs frequency hopping, there is a limit to the number of symbols which are available for the detection of a preamble, and it is necessary to establish symbol timing synchronization with a small number of symbols.
The preamble detection technique which is disclosed in Japanese Unexamined Patent Application Publication No. 2005-039597 seeks the correlation between a fixed preamble pattern and a received signal and detects a peak value in the correlation result as a symbol timing. However, there is a possibility of error detection that a signal different from a preamble signal is detected as a preamble signal, which causes lower throughput of a receiver. This is described in detail hereinafter with reference to FIGS. 18 and 19.
FIG. 18 shows the concept of the technique of implementing the detection of a preamble by detecting a peak value in a correlation result between a fixed preamble pattern and a received signal. As shown in FIG. 18, the technique seeks the correlation with a fixed pattern for every received signal (burst packet 200 in FIG. 18) to obtain a correlation result 107. It then determines a peak value from the correlation result 107, using that the correlation result 107 becomes a peak value when a received signal and the fixed pattern match, that is, when they are synchronized. Specifically, the technique compares the correlation result 107 with a preset default peak detection threshold and detects the correlation result 107 which satisfies the conditions that it is larger than the default peak detection threshold, it is the maximum value in the correlation result 107 up to the present, and there is no larger correlation result 107 in a subsequent predetermined period (i.e. a detection window which is opened according to a symbol length, or a peak detection range in FIG. 18). Upon detection of the peak value, it ends the peak value detection by activating a peak detection end signal 103 in FIG. 18, assuming that symbol timing synchronization is established. Because the peak value is detected at the timing B0 in the example of FIG. 18, the timing B0 is determined as a symbol timing.
After that, the technique opens a detection window at every position where a distance from B0 is a multiple of symbol length and compares the correlation result 107 at the position of each detection window (a boundary detection timing 105 in FIG. 18) with a boundary detection threshold (the multiplication product of a peak value by a predetermined coefficient, which is 0.5 in the example of FIG. 18).
Then, if the correlation result falls below the boundary detection threshold at a certain boundary detection timing 105, the technique obtains the boundary detection timing 105 which is immediately before the certain boundary detection timing 105 as a boundary timing between a preamble signal and data. In the example of FIG. 18, the correlation result 107 at the detection window positions B1 to B3 are greater than the boundary detection threshold, and the correlation result 107 at the position A falls below the boundary detection threshold. Thus, the timing B3, which is immediately before the timing A, is determined as a boundary between a preamble signal and data.
A detection result according to this technique is discussed hereinafter with reference to a received signal (a burst packet 300) shown in FIG. 19.
In this case, a receiver receives another signal (non-preamble signal in FIG. 19) prior to receiving a preamble signal. The correlation result 107 is determined for the non-preamble signal also. For example, consider the case where a peak value is detected at the timing C0. According to the above-described technique, a symbol timing is determined upon detection of the peak value. However, because the timing C0 is for the non-preamble signal, an incorrect symbol timing is determined, and the detection window is opened on the basis of the incorrect symbol timing. Accordingly, the comparison between the correlation result 107 and the boundary detection threshold is performed at the timings C1, C2, C3 and C4. Because the boundary detection threshold is also determined by the correlation result 107 at the timing C0, the timing C2 is determined as a boundary timing between a preamble signal and a payload when the correlation results at the timings C1 and C2 are greater than the boundary detection threshold and the correlation result at the timing C3 is smaller than the boundary detection threshold, thus resulting in error detection.
Further, because a received signal level can change upon deterioration of the receiving environment such as under multipath environment or before performing AGC, it is necessary to set a low default peak detection threshold in order to prevent the omission of detection of a peak value. Therefore, the correlation result which is larger than a default peak detection threshold is likely to appear in a non-preamble signal also, which causes the above problem.
Furthermore, because a peak value at a symbol timing which is determined from a non-preamble signal is not large enough with respect to the correlation result in the other timings, it is highly likely that the correlation results at the positions other than the true symbol positions D1 to D4 also exceed the boundary detection threshold which is set based on the peak value. In the example of FIG. 19, the correlation results at the timings C1 and C2, which are not symbol positions, exceed the correlation result at the timing C0, resulting in error detection of a preamble based on the misdetection of a periodicity.
In addition, once a symbol timing is determined, the above-described technique does not detect a peak value in the period other than detection windows (i.e. the interval between two adjacent boundary detection timings 105; e.g. between C1 and C2, between C2 and C3 etc. in FIG. 19). Accordingly, it is unable to correct the wrongly determined symbol timing C0 in spite of the fact that the correlation result 107 at the true symbol timing (i.e. the timing D1) is greater than the correlation result 107 at the timing C0.
As one approach to avoid the above problem, there is a technique of keep detecting a peak value in the period other than detection windows even after a symbol timing is determined so as to check if there is a correlation result which is greater than a correlation result at a symbol timing and, if it exists, performing update of a peak value and redetection of a symbol timing. However, because the chances for the checking are small in a received signal having a small number of symbols available for symbol timing synchronization like MB-OFDM, it is likely to fail to find the error detection of a symbol timing.
The error detection of a symbol timing leads to error detection of a preamble, which causes lower transmission throughput.